Apr 3, 2009

Unicolonial ants, such as these Argentine ants (Linepithema humile), are genetically unrelated but will cooperate to defeat a much larger adversary.Source: Alex Wild / Live Science

It has been a mainstay of evolutionary theory since the 1970s. Natural selection acts purely on the level of the individual and any cooperation observed between organisms merely hides a selfish genetic motive. There have been two pioneering theories to explain cooperation in the natural world given this framework: the first was William Hamilton's (1964) theory of kin selection and the second was Robert Trivers' (1971) theory of reciprocal altruism.

However, both of these scenarios break down where it comes to unicolonial ants. In a new paper in the journal Trends in Ecology and Evolution(subscription required)Heikki Helantera, of the University of Sussex, and colleagues at Rice University have investigated how previous theories to explain cooperation don't apply for these unique supercolonies.

Unicolonial ants carry polydomy [multiple nests in a supercolony that all individuals rotate through] and polygyny [multiple queens in one nest] to extremes. Colonies are huge, each being a network of hundreds or thousands of nests, each with multiple queens. There is no worker aggression, and there is free movement among nests on a vast scale. The energy that might have been put into fighting and territoriality flows into the common good, more ants.

Such a concept, a form of genuine anarchism in the animal world, was thought to be impossible given existing theory. These ants live in colonies where relatives exist but, with so much migration throughout a network stretching thousands of kilometers, each ant worker is mostly surrounded by total strangers that share none of their genes. Only one other species has ever been known to organize themselves in such a fashion (and if you're reading these words right now you know who you are).To understand how unicolonial ants have come to be the way they are, we must first understand what they're not. Kin selection has proposed that cooperation will emerge in groups that are made up of close relatives. Hamilton's rule, beautiful in its simplicity, proposed that cooperation occurs when the cost to the actor (C) is less than then the benefit to the recipient (B) multiplied by the genetic relatedness between the two (r). This equation is written out simply as rB > C.

To put this into context: an alpha male lion and his brother share half of their genes, so have a genetic relatedness of 0.5. Suppose this brother recognizes that the alpha male is getting old and could easily be taken down. If so, the brother could potentially have eight additional cubs (just to pull out an arbitrary number). But, instead, that brother decides to help the alpha male to maintain his position in the pride and, as a result, the alpha ends up having the eight additional cubs himself while the brother only has five. The brother has lost out on 3 potential cubs. But, even so, because he assisted his brother he has still maximized his overall reproductive success from a genetic point of view: (0.5) x 8 = 4 > 3. He could have attempted to usurp his brother and, perhaps, had the eight cubs himself but he wouldn't have been in any better of a position as far as his genes were concerned.

Reciprocal altruism follows this same basic idea, but proposes a mechanism that could work for individuals that are unrelated. In this scenario, cooperation occurs when the cost to the actor (C) is less than the benefit to the recipient (B) multiplied by the likelihood that the cooperation will be returned (w) or wB > C. This has been demonstrated among vampire bats who regurgitate blood into a stranger's mouth if they weren't able to feed that night. Previous experience has shown the actor that they're likely to get repaid if they ever go hungry one night themselves. This theory requires that individuals be part of a single group, with low levels of immigration and emigration, so that group members will be likely to encounter each other on a regular basis.

Previously, it was argued that all ants followed an extreme form of kin selection. Because of their unique process of reproduction females develop from fertilized eggs and have paired chromosomes (that is, one from each parent). However, males develop from unfertilized eggs and only have a single chromosome from their mother. As a result, female workers share up to 75 percent of their genes with sisters but only 50 percent with their mother (or their own offspring, if they were to reproduce). Worker ants therefore have greater genetic success by not reproducing but, instead, helping to raise and protect their legion of closely related sisters.

This explanation has been somewhat clouded given more recent evidence that queens engage in polyandry (mating with multiple males). A queen will frequently mate with up to five different males and store their combined sperm, around 100 million of them, in a special compartment called the spermatheca. By releasing a single sperm at a time the queen can control the number of eggs she lays. However, because there are multiple fathers, the genetic relationship between the female worker ants is reduced. Female workers may therefore only be related by 25 percent with the females they're helping to raise. Why would female workers continue to be non-reproductive and help rear distant relatives when they could have twice the reproductive success by having their own offspring? While there are strategies female workers employ to maximize their own reproductive success (like preferrentially rearing eggs that they are more closely related to or, in some rare cases, reproducing themselves) it still remains puzzling why ants have been so successful given this seeming contradiction.

If you add to this the realities of multiple queens in a single nest (polygyny) and supercolonies that are composed of thousands of such nests (polydomy), the problem becomes insurmountable. If worker ants share zero percent of their genes with those they're cooperating with, as is the case in these unicolonies, then why cooperate? What do they have to gain?

This is the problem that Helantera and colleagues are seeking to understand in their latest paper. While the authors emphasize a range of possible explanations, I want to focus on just one that has been generating a great deal of interest in the last few years: group selection.

The extreme cooperation of unicolonial ants has been suggested to be an example of selection occurring on levels higher than the individual, such as the superorganism, group or even population.

Group selection is the idea that, under certain circumstances, genes will be selected for because they benefit the overall success of the group rather then simply the individual. While it is usually assumed that these populations will have a high level of relatedness (making the promotion of the group an extended form of kin selection) the authors suggest a scenario in which group selection could apply even among unrelated group members.

This is a possibility I like to call Ronald Reagan's Alien Invasion Hypothesis. In a speech before the United Nations on Sept. 21, 1987 Reagan stated that:

In our obsession with antagonisms of the moment, we often forget how much unites all the members of humanity. Perhaps we need some outside, universal threat to make us recognize this common bond. I occasionally think how quickly our differences worldwide would vanish if we were facing an alien threat from outside this world.

So under this possibility a common threat to all colony members would outweigh the low level of genetic similarity because, unless everyone pulls together, the entire group is in jeopardy. If one colony was competing with a rival colony then selection for individual selfishness could drive the population to extinction while selection for cooperation would allow the colony to thrive.

Under this view, extant unicolonial populations are the ones that have not yet succumbed to selfishness. Relatedness and mutual policing select against selfishness in non-unicolonial populations, but stop applying when relatedness decreases to zero. . . [However], constraints arising from the natural history of the species or pleiotropic effects of selfish genes, might prevent selfish genotypes from arising even under zero relatedness.

This cooperation could then continue long after the initial threat was gone under the force of phylogenetic inertia. Perhaps, in the future, selection would cause the unicolony to break into smaller, more genetically similar colonies once the impetus for group selection no longer exists? Or perhaps the benefits of cooperating with strangers simply outweighs the costs of competition and natural selection has produced a genuinely altruistic society?

Unicolonial cooperation has inspired activist art such as this print from the Beehive Collective.Source: Beehive Collective

At the current time there are 31 known unicolonial ant populations around the globe. This is a small minority given the more than 12,000 described species. However, given that research on unicolonial ants is so new, there is still a great deal of research that needs to take place concerning this unique experiment of the natural world. At the very least, unicolonies provide us with a source of inspiration and the ability to marvel at the amazing beauty and diversity of the natural world. With the knowledge that stable supercolonies composed of strangers continue to thrive in nature, perhaps there's something we could learn from those creatures that first invented this approach.

References:

Hamilton, W.D. (1964). The genetical evolution of social behaviour I and II. — Journal of Theoretical Biology7: 1-16 and 17-52

Helantera, H., Strassman, J.E., Carrillo, J., Queller, D.C. (2009). Unicolonial ants: where do they come from, what are they and where are they going? Trends in Ecology and Evolution. doi:10.1016/j.tree.s009.01.013

Mar 31, 2009

Anup Shaw over at Global Issues has collected an exhaustive collection of recent analysis on the loss of biodiversity in the last few years.

As I wrote in my recent post Rivalry Among the Reefs, the loss of up to 1/3 of coral reefs in recent years could result in unprecedented extinctions of ocean biodiversity.

While occupying only 0.2 percent of the world’s oceans, coral reefs sustain 25 percent of species diversity; an oceanographic public works project that has been in existence for 3.5 billion years. . . Current estimates are that one-third of the world’s coral reefs are in imminent danger of extinction. In an international survey of these most diverse ecosystems in our oceans, researchers determined that global climate change is increasing the average temperature of the Earth’s oceans. This is killing the photosynthetic algae that has adapted into a pristine symbiotic relationship with their hosts. Coral bleaching on a global scale is the result and mass extinction will be the inevitable conclusion unless this trend is reversed.

But loss of biodiversity in the oceans is only one region currently experiencing crisis. The collection of studies and warnings from experts around the world that Anup has gathered are truly staggering. See below for a sample of some of what he posts:

Already resources are depleting, with the report showing that vertebrate species populations have declined by about one-third in the 33 years from 1970 to 2003. At the same time, humanity’s Ecological Footprint—the demand people place upon the natural world—has increased to the point where the Earth is unable to keep up in the struggle to regenerate.

The world environmental situation is likely to be further aggravated by the increasingly rapid, large scale global extinction of species. It occurred in the 20th century at a rate that was a thousand times higher than the average rate during the preceding 65 million years. This is likely to destabilize various ecosystems including agricultural systems.

If current estimates of amphibian species in imminent danger of extinction are included in these calculations, then the current amphibian extinction rate may range from 25,039–45,474 times the background extinction rate for amphibians. It is difficult to explain this unprecedented and accelerating rate of extinction as a natural phenomenon.

Junk-food chains, including KFC and Pizza Hut, are under attack from major environmental groups in the United States and other developed countries because of their environmental impact. Intensive breeding of livestock and poultry for such restaurants leads to deforestation, land degradation, and contamination of water sources and other natural resources. For every pound of red meat, poultry, eggs, and milk produced, farm fields lose about five pounds of irreplaceable top soil. The water necessary for meat breeding comes to about 190 gallons per animal per day, or ten times what a normal Indian family is supposed to use in one day, if it gets water at all.

Mar 29, 2009

The Reef Tank is currently hosting my new post that tells the story of one of the largest controversies in the history of science. It involves Charles Darwin, a son defending his father's honor and the threat of nuclear annihilation. Intrigued? Go over and check it out, as well as some great posts by other fellow science bloggers. Here is a quick taste:

It took the threat of nuclear annihilation between the two greatest powers of the 20th century to solve one of the most profound scientific controversies of the 1800s. In 1952 Dr. Harry Ladd, a researcher for the US Geological Survey, convinced the US War Department to drill holes deep into the Bikini and Eniwetok Atolls just prior to their obliteration by hydrogen bombs. The reason for the drilling had little to do with the nuclear tests as part of Operation Crossroads, but was simply to conduct an experiment based on the hypothesis of coral reef formation first proposed by Charles Darwin in 1837.

Mar 25, 2009

This mountain gorilla has been drinking alcoholic sap fermented in bamboo shoots in the highlands of Rwanda. The next day the nature photographer, Andy Rouse, noticed that the gorillas were a bit worse for wear.

When I went back the next day, it was all very quiet, as if they were nursinggorilla-sized hangovers.

Mar 24, 2009

Could the bonobo homeland also be the origin of our common ancestor?Image: Cyril Ruoso / Time

In an interview with Dan Harris on Nightline, bonobo researcher Dr. Bila-Isia Inogwabini recently suggested that humans may have first evolved in the Western area of the Democratic Republic of Congo rather than in East Africa where most of the fossil evidence has been discovered.

“I really strongly feel that people may have evolved from this region,” he said. "It’s a big claim, yes, I understand, but I really think it is worth it to put it on the table."

Genetic evidence suggests that humans, bonobos and chimpanzees shared a common ancestor between 4.6 to 6.2 million years ago (mya). One population eventually became the genus Homo while the other, Pan, diverged again about 1.3 mya to become modern day bonobos and chimpanzees. What remains unknown is where and why this Pan-Homo split occurred that sent our species along such a different evolutionary path.

The vast majority of hominin fossils have been found in eastern Africa, in what is modern day Ethiopia, Kenya and Tanzania. The most reasonable assumption has long been that it was in East Africa, near the Rift Valley, where the Pan-Homo divisiontook place. However, the earliest hominin fossil find to date, Sahelanthropus tchadensis, is between 6-7 mya and was found in in the central African country of Chad (about 2,500 km away). S. tchadensis was much more ape-like than later Australopithecines and is thought to have coexisted alongside our common ancestor with chimpanzees and bonobos.

This early Chad find is important considering that the fossil record is so sparse between 12 mya and 4 mya (the crucial period as far as we're concerned). The only representatives of this vast period after S. tchadensiswas Orrorin tugenensis (6 mya from Kenya) an ape species that may have been partially bipedal, Ardipithecus ramidus kadabba(5.5 mya from Ethiopia) and Ardipithecus ramidus ramidus (4.4 mya also from Ethiopia). All of these species were much more apelike than the Australopithecines, which first appear in the fossil record 4.15 mya in Kenya and 4.1 mya in Ethiopia. An excellent timeline at hominin.net helps put these dates and the location of the finds into context.

Fossil map showing many of the hominin fossil discoveries throughout Africa.Image: National Geographic

While Ethiopia or Kenya would seem to be the most likely candidates for the split between Homo and Pan, the fact that the earliest known hominin has been found in Chad has raised some questions about this. Writing in the journal Nature about how the discovery of S. tchadensis changes our understanding of early human origins, Brunet. et al state:

This suggests that an exclusively East African origin of the hominid clade is unlikely to be correct. It will never be possible to know precisely where or when the first hominid species originated, but we do know that hominids had dispersed throughout the Sahel and East Africa by 6 Myr.

Furthermore, Australopithecusbahrelghazali (about 3.6 mya) was also discovered in Chad, suggesting that there is a great deal of material still to be discovered in this region.Later hominin fossils have also been discovered in South Africa (such as Australopithecus africanus and Paranthropus robustus). This means that the distribution of hominins extends all the way from Chad to East Africa to South Africa. Interestingly, the nearest central point between all these locations is the Democratic Republic of Congo.

This brings us to bonobos, the species that Dr. Inogwabini has studied for many years. Bonobos are found only in the DR Congo and had their last common ancestor with chimpanzees about 1.3 mya. Emory University primatologist Frans de Waal has argued that bonobos haven't changed much in the 4-6 million years since humans, chimpanzees and bonobos shared a common ancestor.

"The bonobo may more closely resemble the common ancestor of all three modern species," De Waal says. "It's an important issue that's yet to be resolved."

Remarkably, many of these traits are shared by only one other extant species: humans. Regular bipedalism, face-to-face mating (requiring a more ventral orientation of the vagina), reduced limb and body proportions, reduced canines, greater breadth of diet, larger group sizes and reduced competition within groups; all of these traits are shared more closely with humans than chimpanzees. Anatomically, bonobos show more similarities than chimpanzees to the early hominin Ardipithecus (5.5 mya). This could mean that bonobos are closer to the ancestral population and that chimpanzees diverged in order to adapt to different environmental pressures. But it could also be that early humans and bonobos experienced convergent evolution based on similar environments. At this point the evidence to address these questions is thin.

Bonobos are the only primate, other than humans, that regularly walk upright.Image: Unattributed

Unfortunately, fossils are unlikely to help. Rain forest soils are notoriously bad for fossilization. The bones will decay long before minerals can replace the organic material. Even in ideal conditions (such as arid or anoxic environments) fossilization is extremely rare. So there is a certain amount of "environmental bias" in the fossil record. Unless an organism had a large enough range to be living in the right location for fossilization to occur, there will be no record that they ever existed. This could mean that the hominin fossils we do have were from individuals after they had migrated to the far edge of their original range and that the really exciting evolutionary events occurred in Central Africa. If this is the case, then the reason we have so many fossils from East Africa isn't because that was the cradle of humanity, it's just because the conditions were right for fossilization. Unfortunately, without evidence to test this hypothesis it remains in the realm of mere conjecture.

One piece of evidence that makes me think bonobos and humans might have shared more than just a similar environment has to do with a region of DNA promoting the release of oxytocin. At the AVPR1A gene both humans and bonobos (but not chimpanzees) share a repetitive microsatellite locus that Elizabeth Hammock and Larry Young have shown to be important for cooperation, empathy and social bonding. It is far more parsimonious that chimpanzees lost this repetitive microsatellite than for both humans and bonobos to independently develop the same mutation.

So if I had to make my best guess, I would put my money on the Pan-Homo split occuring in the mosaic environments of Central Africa near DR Congo. I would also predict that this common ancestor would appear more bonobo-like than chimpanzee-like. We may never know the real answer. But, considering the exciting "hobbit" fossils discovered on the island of Flores, Indonesia, it may be possible for fossilization to occur even in the rain forest if conditions are just right. At this moment, somewhere in a cave near Lac Tumba in the Democratic Republic of Congo, our common ancestor with bonobos and chimpanzees may be lying in wait for the next intrepid explorer to unearth. If so it would be the anthropological find of the century. Our long search to understand human origins would finally be at an end.

Mar 22, 2009

A new Nightline report travels to the war ravaged Democratic Republic of Congo to interview Bila-Isia Inogwabini, the World Wildlife Fund researcher who discovered an unknown population of nearly 2,300 bonobos. Considering that some researchers estimate there to be fewer than 10,000 bonobos alive in the wild, this discovery was hailed as a major development in the effort to save the species from extinction.

I was introduced to Bila a few years ago while working on my article Behind Enemy Lines for Wildlife Conservation. At the time he was just preparing his expedition to the Lac Tumba region of Western Congo. His team's findings were subsequently published in the Cambridge University journal, Oryx (subscription required).

Mar 20, 2009

As reported in the latest edition of Nature, government officials in Turkey have just censored the leading science periodical, Bilim ve Teknik (Science and Technology) for placing Charles Darwin on the cover. The editor was subsequently fired by the government agency that supports the magazine and many Turkish scientists are justifiably outraged.

In Turkey, as in many countries, the civil service is expected to mirror the ruling party's ideology. So, although they are keen funders of research, most senior government officials, in common with most of the population, do not believe in evolution by natural selection. The education minister Hüseyin Çelik, for example, has proclaimed his belief in intelligent design.

This is unfortunate considering that Muslim scholars and scientists were at the forefront of the scientific revolution long before Europeans got their act together. It's only been in the last twenty or thirty years that there has even been a crackdown on evolutionary ideas in Turkey. According to the History of Science Society:

In the 1970s, political Islam started to gain strength in Turkey as well as the rest of the Muslim world. Evolution became a minor culture war item, as a way for Islamists to demonstrate opposition to secular life without taking the risk of naming official secularism as a target. But creationism came into its own only in the mid 1980s, when in the aftermath of a short period of military dictatorship, religious conservatives gained control of the Turkish Ministry of Education. These conservative Muslims thought evolutionary ideas were morally corrosive, yet they found themselves in an environment where science commanded significant cognitive authority. So they needed a way to suggest that evolution was a fraudulent, scientifically dubious idea. They found the resources they needed in American “scientific creationism,” and invoked Christian creationists in a curious mirror image of the way Turkish secularists regularly relied on Western scientific authorities.

So there you have it. In an effort to reject Western secular ideas, fundamentalist Turks have embraced the Western fundamentalist rejection of science and reason. You wouldn't think that the very people that are most vocal about promoting war in the Middle East (isn't it curious how those most apt to honor the "prince of peace" are so ready to go to war) would also be the ones that conservative Muslims would be listening to for their approach on science and education. I guess that just shows that there is common ground between seemingly incompatible societies (though it doesn't offer us much hope at present).[Read more →]